Snowfall: have canal rerouted (9)
I believe the answer is:
avalanche
'snowfall' is the definition.
(I've seen this before)
'have canal rerouted' is the wordplay.
'rerouted' indicates anagramming the letters (I've seen this in other clues).
'have'+'canal'='havecanal'
'havecanal' anagrammed gives 'AVALANCHE'.
(Other definitions for avalanche that I've seen before include "Mass of snow falling down a mountain" , "flood" , "Slide of large amounts of snow or stone" , "Mountainside fall" , "Slide of a large amount of snow down a mountain" .)